The invention relates to an apparatus and method for treating a flowing suspension. Liquid is continuously extracted from the flowing suspension; if the extraction is combined with the introduction of a treatment liquid, then treatment of the suspension occurs, while if the extraction is not accompanied by an introduction of treatment liquid, dewatering of the suspension occurs.
When used for dewatering, the apparatus according to the present invention is of the same general type as the dewatering apparatus shown in U.S. Pat. No. 4,041,560 (the disclosure of which is hereby incorporated by reference herein) wherein a screen body is reciprocated in a direction generally parallel to a suspension flow, with backflushing of the screen openings taking place during movement of the screen opposite to the direction of movement of the suspension. While prior art systems such as disclosed in U.S. Pat. No. 4,041,560 are generally successful in accomplishing their desired result, there are several areas where improvement is possible. The apparatus according to the present invention is advantageous compared to such prior art assemblies in that the backflushing takes place substantially evenly over the whole of the screen surface. The apparatus and method according to the present invention are also advantageous over prior art systems in that they facilitate the introduction of a treatment liquid into the flowing suspension, the treatment liquid introduction being practiced so that during liquid extraction with the screen body a crosswise flow of treatment liquid is obtained through substantially all suspension flowing through the container. This makes the apparatus and method according to the present invention extremely versatile for performing a variety of treatment functions.
The apparatus and method according to the present invention may be used for dewatering or treating a wide variety of suspensions, but are particularly useful for dewatering and treating comminuted cellulosic fiber material suspended in liquid, which liquid can either be water or a chemical treatment liquid, such suspensions commonly being used and produced during the production of paper products.
According to one aspect of the present invention, apparatus for continually extracting liquid from a flowing suspension is provided. The apparatus includes an elongated container through which suspension to be treated flows in a first direction, an inlet and an outlet being provided adjacent opposite ends of the container in a dimension of elongation thereof. Screen means are provided for removal of a portion of the liquid from the suspension flowing through the container, and extending generally in the direction of elongation of the container. The screen means include a perforated screen surface defining a screen cavity volume therewithin. An outlet from the screen cavity volume is connected to an area remote from the container, and bearing means mount the screen means for movement generally in the dimension of elongation of the container, and means are provided for moving the screen means in the dimension of elongation of the container, both in the first direction and in a second direction opposite the first direction. Baffle means are disposed within the screen cavity volume for cooperating with the screen surface so that backflushing of the screen surface takes place during movement thereof in the second direction, the backflusing being substantially even over the whole of the screen surface.
According to another aspect of the apparatus according to the present invention, the screen means includes a screen surface defining a screen cavity volume therewithin, the cavity volume tapering inwardly in the first direction so that it has a larger cross-sectional area at the points closer to the container inlet than at points closer to the container outlet. According to both described aspects of the present invention, preferably the screen surface tapers inwardly so that it is spaced farther from the container interior sidewalls at points closer to the container outlet than it is at points closer to the container inlet.
The container preferably is substantially circular in cross-section, having a substantially constant diameter along the dimension of elongation thereof, and treatment liquid introduction is provided by annular baffle plates disposed at at least one position along the inward taper of the screen body at the container interior sidewall. The screen moving means preferably moves the screen means at a first speed comparable to the speed of flow of suspension in the first direction, and at a second speed, much greater than the first speed, in the second direction. Withdrawal of suspension from the container outlet may be facilitated by a rotating scraper.
According to an exemplary method of the present invention, treatment of the suspension flowing through a container in a first direction is provided. The method comprises the following steps: Introducing suspension under a motive force into the container inlet and withdrawing it from the container outlet so that it flows in the first direction within the container. Effecting movement of the screen body in the first and second directions within the container to extract some of the liquid from the moving suspension within the container so that it flows into the volume defined by the screen body. Withdrawing liquid passing into the volume defined by the screen body from that volume to an area remote from the container; and introducing treatment liquid into the container, from an interior sidewall of the container, at at least one position along the inward taper of the screen body, so that during liquid extraction with the screen body a crosswise flow of treatment liquid is obtained through substantially all suspension flowing through the container.
The introduction of treatment liquid may be practiced utilizing an annular baffle plate provided at an interior sidewall of the container, the treatment liquid being introduced so that the liquid initially has a velocity component in the first direction and is introduced substantially evenly around the circumference of the container. Alternatively, or additionally, treatment liquid introduction pipes may be disposed at various points within the container, this being especially useful where a plurality of screen surfaces are provided, or the suspension flows both centrally and exteriorly of an annular screen surface.
It is the primary object of the present invention to provide an apparatus for continuously extracting liquid from a flowing suspension with substantially even backflushing of a screen surface thereof, and for providing the effective introduction of treatment liquid into a flowing suspension in such a device. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.